Jem changes as many young boys will over the two and one-half years in which To Kill a Mockingbird takes place. Jem is nine years old when the novel begins, and he is a boy full of mischief and who likes to play games with his closest friend, his sister, Scout. When Dill Harris arrives to spend his first summer in Maycomb, the Finch children quickly accept him as the new member of their triumvirat. They spend most of their time wrapped up with the mysterious Boo Radley--trying to figure ways to make him come out of his house, and acting out imaginary dramas of the Radley family. First believing the town gossip about Boo's nocturnal habits of peeping and animal carnage, Jem soon comes to realize that Boo is their friend, albeit a silent and unseen one, when gifts are left behind in a knothole of the Radley oak tree. Jem understands that the gifts could have come from no one but Boo, and he and Scout eventually abandon their original goals, and leave Boo to his self-imposed privacy.

By the second half of the novel, Jem is 12 and approaching puberty. He is growing hair under his arms, and he has little time for his younger sister anymore. Jem is transfixed by the approaching trial of Tom Robinson, and he is devastated when Tom is found guilty. Jem recognizes that the jury was biased from the start and that they had deliberately disregarded the evidence that his father had presented to them. It is just one case of lost innocence he experiences during the novel, leaving him more wary of adults and their ideas of justice. By the night of the Halloween pageant, Jem no longer believes in the superstitions so important to him when he was younger; nor does he run past the Radley house out of fear. When Bob Ewell attacks him and Scout, Jem gallantly attempts to protect his sister. He is left with a broken elbow, which we find out (through Scout's retrospective view on the first page of the novel) has little effect on his athletic career.

... he was seldom self-conscious about his injury. His left arm was somewhat shorter than his right; when he stood or walked, the back of his hand was at right angles to his body... He couldn't have cared less, so long as he could punt and pass.